Murdered baby Preston Davey’s mum was once branded one of Britain’s most notorious teen killers after torturing a gran and dumping her body in a canal. As Jamie Varley faces life behind bars for the horrific murder of 13-month-old Preston, it can now be revealed the tot’s mum Sarah was just 14 when she and a pal beat frail Lily Lilley, 71. The pair befriended lonely Lily at her home in Failsworth, Greater Manchester . After being invited in for a cup of tea, they taunted her, squirted her with